Electric motors utilize a variety of protectors to avoid degradation of the winding insulation during abusive locked rotor conditions. Permanent magnet motors applied in the automotive industry utilize bimetallic or polymer PTC protectors mounted on the brush card, which are connected in series with the motor windings. This arrangement promotes detection of elevated locked rotor versus normal running currents and increasing ambient temperature within the motor housing. The combination of internal I2r heating and increasing ambient temperature drives the protectors to interrupt the electric circuit which limits the winding temperature to an acceptable level.
An example of a prior art polymer PTC protector particularly adapted for use with a 14 Vdc window lift motor application is shown in FIG. 1 in which a thin, e.g., approximately 0.010 inch thick, polymer chip 1 having metal foil current collectors 1a on opposite face surfaces is sandwiched between and soldered to relatively thick, e.g., 0.031 inch thick, copper or brass terminals 2 to produce the correct trip time response curves. The thick terminals are used to heat sink the polymer PTC chip during transient locked rotor conditions to extend initial trip times at elevated ambient temperature (reference 80° C.) to avoid nuisance tripping. The current sensitivity of the chip is designed to work with the motor's increasing internal ambient temperature during fixed locked rotor conditions to keep the winding temperature below 250–300° C. Trip times at low voltage, low ambient and low current commutation typically take several minutes so that increasing internal ambient temperature is relied on to trip the polymer PTC chip.
The winding temperature of proposed 42V automotive operating system motors can increase 300° C. in 10 seconds due to design modifications required for normal operation at 42 Vdc. As a result, protectors cannot utilize the motor's internal ambient temperature to drive the tripping action to be effective since the accelerated winding's temperature rise will cause the winding insulation to melt prior to raising the motor protector's temperature mounted on the brush card.
Protectors made for use with 42V motors must contend with ampere levels decreased by a factor of three for similar power applications, compared to 14 v systems. This promotes increasing resistance of the polymer PTC chip by a factor of nine to produce similar I2r current sensitivity and/or reducing the chips mass.
With respect to Polymer PTC solutions, as alluded to above, the reduction of cross sectional area to achieve resistance requirements results in tripping the protector nine times faster during overload conditions due to increased rates of temperature rise. This also results in nuisance trip issues during transient locked rotor conditions. Several motor manufacturers specify minimum trip time requirements (i.e., 20 seconds) during transient locked rotor or high torque conditions; allowing applications such as window lift motors to drive the glass into the seal for a specific time duration or number of up and down cycles. Increasing the polymer PTC thickness and reducing the cross sectional area by a factor of three would provide reduced rates of temperature rise and increase current sensitivity. However, the cost of effectively blanking polymer PTC chips with proposed diameter to thickness ratios would be difficult with existing manufacturing technology. In addition, locked rotor to motor run current ratios are greater in 42V systems requiring further reduction in the polymer PTC's rate of temperature rise to avoid nuisance trips.
Another complication relates to the phenomena of the polymer PTC experiencing torque performance degradation wherein the PTC resistance increases by some 40% after the initial switch and reset operation of the PTC element. It is postulated that this is caused by carbon particles in the polymer not achieving 100% realignment. The resistance shift can be even greater than 40% immediately after the supply voltage is removed producing greater transient motor performance degradation and nuisance trip conditions. Thus, safety applications must be made with the polymer PTC in its lower resistivity state producing the lowest level of I2r heating and nuisance trip analysis must be performed with the polymer PTC in its highest resistivity state.